Generally, most magnesium alloys have a higher bare corrosion rate than aluminium. Therefore, the role of surface protection is very important in design of magnesium aircraft components.
FP6 project AEROMAG showed that modern surface treatment technologies, such as PGA ALGAN 2M, MAGOXIDE, MAGPASS and Gardobond X4729 offer high protection level for magnesium components. The results were very close to those of coatings on aluminium. It is well-known that magnesium has galvanic corrosion with any other metal. Therefore it's impossible to use metallic layers on magnesium as conductive coatings. The option was to build thin chemical conversion layers which have low electrical resistance.
For many years, such conductive coatings were built on base of Cr VI compounds. The examples of such treatments are Dow-19 and NH35. Due to environmental restrictions, Cr VI shall be replaced. Chemical conversion coatings on the base of Cr III are marked as potential replacement of Cr VI processes on aluminium. However, performance of Cr III processes on magnesium is very low. Modern alternative to Cr VI treatments are silanes. Silane protective coatings are based on ability of hydrolysed silanes to build bonds with metallic substrate. Silane treatments are environmentally friendly and cost effective due to low concentration of active compound (usually 1-5%), short process sequence, low energy and water consumption and they do not have galvanic corrosion with magnesium. They can be applied by immersion, spraying or locally (touch up). The important advantage of silane coating technology is precise control for coating thickness.
Therefore, in order to approach high corrosion resistance with electrical conductivity, the development was focused on silane structure rather than on coating thickness. In the frame of the development, structure of proprietary silane was modified to increase number of hydrolysed groups and to introduce additional, chemical stable functional groups.
The main objective of this project was development and implementation of high corrosion resistance conductive coating for magnesium sheets in A/C. This coating should provide minimal bare corrosion requirement for stand-alone conductive coating – cycling salt spray test in accordance with EUROCAE ED14/ RTCA DO160 Section 14 - 48 hours in SST, 48 hours out of SST without rinse. This coating should provide corrosion resistance for electrical bonding scheme - small area of conductive coating on sheet with selective coating – 168 hours in SST (ASTM 117 and MIL-DTL-5541 Class 3). The coating won't influence strength and fatigue parameters of the substrate - high-strength magnesium sheet.